Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 12, 2016
Valid: July 13-17, 2016 (Wednesday-Sunday)

Summary:

There is an unusually high amount of uncertainty in the medium range forecast, particularly for the end of the period, due to questions about the speed of an approaching weak cold front and its associated precipitation. On Wednesday and Thursday, ozone across most of the region will be tempered by diurnal convection. The exception will be along the eastern Mid-Atlantic, where afternoon sunshine will promote rising ozone along the I-95 Corridor, with a Marginal chance for an ozone exceedance both days. There are several questions for Friday, with the models diverging with the placement of the cold front. The trend in today’s guidance is for a slower frontal passage and to keep the front farther north. Neither the GFS nor EC develop convection across the northern Mid-Atlantic on Friday, so at this point in time, it seems more likely that the I-95 Corridor will remain dry through most of Friday, allowing for a day of intense heat and rising ozone. Therefore, the chance for an ozone exceedance will rise to Appreciable for locations along and east of the I-95 Corridor on Friday. There is also uncertainty in the forecast for the weekend, with the potential of afternoon thunderstorms in association with the cold front, depending on its speed. The front now seems likely to stall in vicinity of the Mason-Dixon Line on Saturday and Sunday. Locations adjacent to the front will experience clouds and scattered precipitation, but any areas that remain clear should see some ozone production. As a result, the chances for an ozone exceedance will be Marginal for the weekend.

Discussion:

The weather forecast models are in fair agreement through most of the medium range, but struggle to reach consensus on the passage of a weak cold front and its associated precipitation during the latter half of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level closed low moving eastward through the southern Canadian Prairies will force the upper level ridge, which is currently overhead, northeastward toward the Canadian Maritimes on Wednesday. The mid-level ridge in place across the southeastern U.S. will be slightly suppressed on Thursday as the trough nears the Mid-Atlantic, but it will remain hooked up with the Bermuda High through the entirety of the medium range. On Wednesday, the upper level ridge will fully exit the Mid-Atlantic, returning most of the forecast region to mostly zonal flow aloft. The Canadian closed low will open up into a broad, weak trough across southern Canada/Great Lakes on Thursday, with the axis of the trough shifting slightly southeastward into the Great Lakes region by 18Z, pulling a weak cold front towards the Ohio River Valley. The deterministic models still are not in agreement with the placement and speed of the weak cold front, which is expected to reach the western Mid-Atlantic sometime on Friday morning. This is further complicated by the fact the anemic cold front does not have a distinct wind shift in the guidance or an area of mid-level cooling in its wake. The NAM appears to be faster than both the GFS and EC, bringing the cold front to roughly I-81 by 18Z Friday, which can be seen more clearly in the 06Z 12 km NAM. The WPC considers the NAM’s solution to be an outlier, and so do we, since the trend in the overall model guidance seems to be a slower frontal passage which does not penetrate as far south into the Mid-Atlantic. To that end, the GFS has slowed down in comparison to yesterday’s runs, but it is still faster than the EC, with the cold front reaching the western Mid-Atlantic on Friday morning, and then stalling before moving to roughly the Mason-Dixon Line on Saturday. The EC has a relatively convoluted solution, with its upper level support failing to manifest itself at the surface in the form of precipitation. It appears that it brings the cold front to the MDL by 12Z Saturday, though this is not clearly reflected in its precipitation pattern at the surface. These questions spill over into the weekend, but all the models have the front stalling across the central Mid-Atlantic, somewhere in the vicinity of the MDL or even PA/NJ, with the entire Mid-Atlantic remaining under the influence of the weak upper level trough. On Sunday, the trough will slowly begin to lift northeastward, allowing the mid-level ridge/Bermuda High over the southeastern U.S. to build back northward into the northern and central Mid-Atlantic, pushing the stalled front northward as a warm front.

Surface high pressure will remain off the Mid-Atlantic coast on Wednesday, promoting clear skies and southeasterly surface winds across the eastern portions of the northern and central Mid-Atlantic. 06Z GFS back trajectories are recirculating, but have more of a southeasterly onshore component, which should be sufficient to clean out coastal locations. Depending on the strength of surface winds, this southeasterly flow could transport pollutants just to the north and west of the I-95 Corridor, which would favor ozone formation in locations that receive sufficient afternoon sun. One potential obstacle to ozone formation could be the development of afternoon convection and thunderstorms. Increasing regional humidity and intense heat will effectively destabilize the lower atmosphere throughout the northern and central Mid-Atlantic, increasing the likelihood of convection initiation. The deterministic models are wetter today in terms of afternoon precipitation; they all show precipitation, albeit light, reaching the I-95 Corridor by 00Z Thursday. The 03Z SREF and 06Z 4 km NAM, however, continue to keep most of this diurnal precipitation to the west of I-95 through Thursday afternoon. The 06Z NCDENR air quality model sides with the deterministic models, keeping almost the entire Mid-Atlantic in the Good range on Thursday. The NOAA and BAMS air quality models, however, develop Moderate ozone across PHL and northern NJ, just north and west of the I-95 Corridor, with the NOAA model also developing Moderate ozone across portions of central and western PA and western MD. Regardless, it appears that there will be enough factors in play on Wednesday that should act to temper rising ozone along and near the I-95 Corridor, namely light onshore flow and fairly widespread afternoon precipitation. As a result, the chance for an ozone exceedance on Wednesday will be Marginal, mainly for locations just to the north and west of the I-95 Corridor.

Sweltering heat and tropical humidity is expected on Thursday, with temperatures climbing into the 90s °F across much of the Mid-Atlantic, with dew points in the 70s °F. These conditions will once again support the development of widespread diurnal convection. The deterministic models develop widespread precipitation by 00Z Friday, but keep the heaviest rain to the west of the I-95 Corridor. Back trajectories for the I-95 Corridor shift southwesterly, which should be less clean than the previous day’s transport pattern, essentially pushing emissions directly up the I-95 Corridor. As a result, ozone production is expected along the I-95 Corridor for Thursday, where it should remain clear through the afternoon. The current air quality model runs reflect this, with the BAMS and NCDENR models bringing the I-95 Corridor into the Moderate range. The chance for an ozone exceedance on Thursday will remain Marginal for locations along the I-95 Corridor, assuming that they remain to the east of diurnal convection throughout the afternoon.

As mentioned earlier, there is considerable uncertainty in the forecast for Friday due to questions regarding the exact track of an approaching weak cold front and its associated precipitation. The EC is the slower solution, compared to the GFS, although both deterministic models are slower today overall, and keep the front farther north. Interestingly, the EC does not develop much precipitation along the front, which means that the frontal passage would not be a limiting factor for ozone production along the I-95 Corridor. The GFS is a bit faster than the EC, but it doesn’t develop any precipitation across the northern Mid-Atlantic either. At this point in time, the model trend is to keep the I-95 Corridor clear through Friday afternoon. This solution allows potentially ample ozone production along the I-95 Corridor in response to intense afternoon sunshine, westerly back trajectories, and a period of stagnating surface winds in the morning. The air quality models appear to favor a drier solution as well, with the NC model in particular resolving USG conditions along the I-95 Corridor. Therefore, the chance for an ozone exceedance will rise to Appreciable on Friday, mainly for the I-95 Corridor.

Lingering questions from Friday will continue to impact the forecast on Saturday, but it appears that the cold front should push southward and stall across the central Mid-Atlantic sometime during the afternoon. This will promote clouds and scattered diurnal precipitation in the vicinity of the front. Though the northern Mid-Atlantic may be post-frontal, there will not be much of an air mass change behind the weak cold front. As a result, ozone levels may continue in the Moderate range along the I-95 Corridor. The BAMS and NCDENR models back off ozone on Saturday. Given the uncertainty in the forecast and the possibility of a slower cold front, the chance for an ozone exceedance on Saturday will drop only slightly to Marginal for locations along and east of the I-95 Corridor.

The majority of the Mid-Atlantic will remain under the influence of the weak upper level trough on Sunday. A stalled front across the central Mid-Atlantic will continue to act as a focus for clouds and precipitation, especially during the afternoon. Back trajectories for the I-95 Corridor shift northerly, although they remain relatively short due to the nearby front. There still should be enough afternoon sunshine, however, to encourage some ozone formation along the I-95 Corridor. At this point in time, the chance for an ozone exceedance on Sunday will remain Marginal.

-Brown/Huff